When transmitting images from an emitter (as a server) to a terminal (as a gateway), or when storing images on a storage device, transmitting and/or storing a color transform together with these images is sometimes required. Color transforms need to be applied to images, videos or parts of images, notably when the color coordinates representing colors in these images need to be adapted to another color encoding standard (colorimetric color management), to another color gamut (color gamut mapping), to another viewing conditions (color appearance) or to another artistic intent (color grading). A color transform can modify the hue, the saturation, the white balance, the black value, the contrast, the white level, or other aspects of an image. If such a color transform is not inherently available at the place (ex. gateway) where the images should be color transformed, the color transform need to be transmitted to this place, notably together with the images to be transformed. In the following, this place is called terminal. Such a terminal may be for instance a gateway, a TV set, a tablet or a smartphone. Images and related color transforms are often from the same origin, i.e. from the same emitter. For example, a video on demand service provides images and a related color transform that both are transmitted to a terminal where the color transform is applied to the images before being displayed. In another example, images are exchanged between different post-production facilities. Each image has an associated color transform adapted to display it on a given reference display device having its own color characteristics.
When images and color transforms have to be transmitted together to a terminal, or stored on a storage device, means adapted for their transmission (data channels, files) or storage and sufficient capacity (channel data rate, file system size) are required. This invention addresses a series of situations where the use of color transforms is normally prohibited. In a first situation, an image transmission channel is not adapted to transmit color transform. For example, in a HDMI interface, videos and various metadata can be transmitted, but the specification of the HDMI standard does not include the notion of a color transform. In a second situation, a storage system is not adapted to store color transforms. For example, a video asset management system for video production or at home in a home network does not include the notion of color transform, i.e. video images can be stored in files and retrieved, but no color transform can be stored together with these videos. In a third situation, a proprietary, open system for storage and transmission of videos comprises means implemented to store and to transmit color transforms together with videos, but, for the reason of disc storage shortage and/or limited bandwidth of transmission channels, the storage and/or transmission of color transforms is prohibited.
In a situation of limited bandwidth for transmission of video images or of limited storage capacity for video images, several methods are known to compress the data representing these images in order to match these limitations. In order to control the quality of the received or stored images, the so-called rate-distortion principle is generally used that relates an increasing data rate (less compression) with a decreasing distortion (better quality). It is known how to apply rate distortion theory to efficiently compress images and videos. For example, A. Ortega gives an introduction in his article entitled “Rate-distortion methods for image and video compression” published in the Signal Processing Magazine of the IEEE, volume 15, issue 6, in November 1998.
Compression and transmission of color transforms is a newer field. A color transform can be represented by a Color Look Up Table (CLUT), by a splines-based model, by a Gain-Offset-Gamma (GOG) model or by any other parametric model. For a CLUT for example, the quality of the data representing the color transform can be controlled by the number of CLUT entries. The larger is the number of entries, i.e. the larger is the rate of data, the lower are the approximations of the CLUT, i.e. the lower is the distortion of these data.
There are known methods for image compression employing a color transform. For example, Marpe et al. propose in their publication entitled “AN ADAPTIVE COLOR TRANSFORM APPROACH AND ITS APPLICATION IN 4:4:4 VIDEO CODING” published in the Proc. 14th European Signal Processing Conference (EUSIPCO 2006), Florence, Italy, September 2006, a method to choose one color transform of an image out of a set of given color transforms such that compression of this image yields best rate with minimum distortion is disclosed. In this article, the color transform is applied before image compression. Practically, the index of the chosen color transform would be needed to be stored or transmitted in order to allow correct decoding of the compressed image.
This invention presents a specific method to embed a color transform into an image or a video in a context where the color transform is applied after image compression and decoding.
Known methods exist for embedding data into images, for instance data hiding techniques such as watermarking. These methods are usually used for copy and copyright protection. The aim of image watermarking is to embed, in a secure way, a robust and not perceptive message (such as Gaussian noise) in an image. Even if, in such methods, the data inserted in the compressed images may be themselves compressed, there is no relationship between the compression of images and the compression of the data.